1. Field of the Invention
The present invention relates to a ceramic laminate and a method of manufacturing a ceramic sintered body.
2. Description of the Related Art
In general, multilayer ceramic substrates have been used as components on which active elements, such as semiconductor IC chips, and passive elements, such as capacitors, inductors and resistors, are mounted. Also, multilayer ceramic substrates have simply been used in semiconductor IC packages. Specifically, these multilayer ceramic substrates have been widely used to construct various electronic components including PA module substrates, RF diode switches, filters, chip antennas, various package components and complex devices.
In order to manufacture the above-described multilayer ceramic substrates, dielectric sheets having wiring conductors formed thereon are laminated, and the sintering process is necessarily performed on the laminate to achieve optimum characteristics. However, after this sintering process is performed, the multilayer ceramic substrates shrink because ceramics are sintered. Since multilayer ceramic substrates do not shrink evenly in all directions, dimensional changes occur in the planar direction of ceramic layers. The shrinkage of the ceramic substrate in the planar direction also causes undesirable deformations or distortions. Specifically, the accuracy of external electrodes for connections with chip components, which are mounted onto multilayer ceramic substrates, may be reduced or wiring conductors may be disconnected.
The shrinkage of the ceramic substrate in the planar direction causes a misalignment between conductor patterns and the ceramic substrate when mounting components. As a result, it may be impossible to mount semiconductor chips, such as chip size packages (CSPs) and MCM (multi-chip modules), with high accuracy. Therefore, there has been proposed a so-called non-shrinking method in order to remove shrinkage in the planar direction in a sintering process when multilayer ceramic substrates are manufactured.
According to a general non-shrinking method, constraining sheets are formed using alumina powder, which is a ceramic that is not sintered at 900° C. or less, the formed constraining sheets are laminated on the top and bottom of low temperature co-fired ceramic (LTCC) dielectric sheets to form a ceramic substrate, a predetermined weight is applied to the ceramic substrate to perform plasticizing and sintering, and then the constraining sheets are removed therefrom, thereby obtaining a ceramic substrate. FIG. 1 is a cross-sectional view illustrating one process of a general non-shrinking method of manufacturing a ceramic substrate. Constraining layers 11 are disposed on the top and bottom of a ceramic laminate 10 that has a plurality of ceramic sheets laminated onto one another. Here, each of the constraining layers 11 is not sintered at a sintering temperature of the ceramic laminate 10. The constraining layers 11 can prevent shrinkage in the planar direction of the ceramic laminate 10 during the sintering process.
However, in the non-shrinking method, illustrated in FIG. 1, a large constraining force is applied to ceramic sheets adjacent to the constraining sheets 11, but a relatively small constraining force is applied to the inner part of the ceramic laminate 10. Since the constraining force is unevenly applied to the ceramic laminate 10, a stress imbalance occurs in the inner part of the ceramic laminate 10. As a result, the reliability of the ceramic substrate may be deteriorated. This problem may be worsened when the ceramic laminate 10 is thick.